US Defence Secretary Leon Panetta has lifted the military's ban on women serving in combat roles, potentially opening hundreds of thousands of frontline positions to women.

The ruling, officially announced on Thursday, overturns a 1994 rule barring women from small ground-combat units.

But the military will have until 2016 to argue for any specific posts they think should remain closed to women.

President Barack Obama welcomed the "historic step".

At a Pentagon press conference, Mr Panetta said: "Female service members have faced the reality of combat, proving their willingness to fight and, yes, to die to defend their fellow Americans."

'Valour knows no gender'

He acknowledged a complaint frequently voiced by women denied combat roles - that military careers are hindered by a lack of combat experience.

Analysis

By Paul AdamsBBC News, Washington

General Martin Dempsey and Defence Secretary Leon Panetta made it clear that this move is about recognising reality: women have been serving in combat for years. But while historic, the fact that the two men have rescinded the 1994 rule doesn't herald an era of automatic changes throughout the military.

Gen Dempsey's 9 January memo noted that the time had come to eliminate all "unnecessary" gender-based barriers to service. It follows that there may be plenty of "necessary" barriers. It is now the job of service chiefs, over the next three years, to identify where those barriers lie.

The expectation is that a large proportion of the 237,000 positions now closed to women will open. But even if certain units (special operations, for example) remain off limits, Gen Dempsey says he thinks today's move will have wider benefits, including tackling the military's problem with sexual harassment.

"I fundamentally believe that our military is more effective when success is based solely on ability and qualifications and on performance," said Mr Panetta.

Military chiefs will be asked to report back to Mr Panetta by 15 May on their initial plans to implement the new policy.

A senior defence official said about 237,000 jobs would be newly open to women "who can meet the standards".

President Barack Obama said: "This milestone reflects the courageous and patriotic service of women through more than two centuries of American history and the indispensable role of women in today's military.

"Many have made the ultimate sacrifice, including more than 150 women who have given their lives in Iraq and Afghanistan - patriots whose sacrifices show that valour knows no gender."

Mr Panetta hopes to have the process complete by 2016.

The US is likely to have withdrawn all combat troops from Afghanistan well ahead of that time.

Some jobs are expected to be opened to women this year, while others - including for special forces such as the Navy Seals and the Delta Force - could take longer.

One of the most high-profile female combat veterans in America is US Congresswoman Tammy Duckworth, an Illinois Democrat who lost her legs when the Army helicopter she was piloting in Iraq was shot down.

On Thursday, she quipped on NBC's Today show: "I didn't lose my legs in a bar fight. I'm pretty sure I was in combat."

Restrictions were first eased a year ago, when the Pentagon opened up 14,500 roles, closer to the frontline, which had previously been off limits to female personnel.

In November, a group of four women in the military sued the defence department over the ban, arguing that it was unconstitutional.

One of the plaintiffs, Marine Corps Capt Zoe Bedell, said existing rules had blocked her advancement in the Marines.

During the Iraq and Afghan wars, US female military personnel have worked as medics, military police and intelligence officers, sometimes attached but not formally assigned to frontline units.

As of 2012, more than 800 women were wounded in those wars, and more than 150 have died.

Women comprise 14% of America's 1.4 million active military personnel.